The invention is directed to a process for the production of benzoyl cyanide by reaction of benzoic anhydride with an alkali metal cyanide. Benzoyl cyanide is an important intermediate product for the production of herbicides.
It is known to produce benzoyl cyanide by the action of more than stoichiometrical amounts of copper (I) cyanide on benzoyl chloride. The reaction is carried out at temperatures up to 80.degree. C. in acetonitrile or benzonitrile or in ether with the addition of more than stoichiometrical amounts of lithium chloride or lithium iodide (Normant et al, Bull. Soc. Chim. France (1972) pages 2402-2403) or at temperatures of 220.degree. to 230.degree. C. in the absence of a solvent (Org. Synth. Coll. 3, 112-114). At best these processes give a yield of 65%.
It is also known to convert benzoyl chloride to benzoyl cyanide by means of an alkali cyanide in a two phase system consisting of water and a solvent which is immiscible with water in the presence of a quaternary alkyl ammonium salt (Koenig, Tetrahedron Letters No. 26 (1974), pages 2275 to 2278). In this process the yield only amounts to 60%.
Furthermore, it is known to produce benzoyl cyanide from benzoyl chloride by reaction with water free hydrogen cyanide and an at least equimolar amount of pyridine (Z. Phys. Chem. 192 (1943), 200-201). This process gives yields of 78%.
A disadvantage of these known processes is that there are formed byproducts to a considerable extent, particularly the dimer of benzoyl cyanide (the benzoyloxy-phenyl malonitrile). Consequently, not only is the yield unsatisfactory but also its purity. Benzoyl cyanide can be separated from its dimer only with considerable difficulty and even then only incompletely.
Finally, it is known to produce benzoyl cyanide by reaction of a mixture of benzoic anhydride and potassium cyanide at a temperature of 190.degree. C. (Erlenmeyer, Liebigs Ann. Chem. 287 (1895) 302 et seq., particularly pages 305 to 307). This process merely results in a yield of 10%.